Many modern semiconductor devices are composed of MOS (Metal-Oxide-Semiconductor) transistors and capacitors, in which the MOS transistors generally include a source, drain, and gate. The gate is sometimes called a gate stack because it may include a plurality of components components, such as a gate electrode and an underlying gate dielectric. Sidewall spacers (also called spacers, or spacer layers) may be adjacent to the gate structure and usually include an oxide layer and a nitride layer component.
Spacers serve a number of functions in the formation of semiconductor devices. One function of spacers is to assist in the alignment of silicide layers above the source-drain regions and the gate electrode. Silicide layers are highly conductive compared to the underlying source-drain regions and gate electrode, and facilitate the transfer of electric signals to and from the transistor. The silicide layers are formed by depositing a metal layer (e.g., titanium, cobalt, nickel, etc.) where the metal is reactive with the underlying materials in the source-drain regions and gate electrode, but not as reactive (or not reactive at all) with the materials in the spacers. The unreacted metal is selectively etched to formed gaps between the reacted silicide layers, thus preventing the layers from forming shorts between one another.
Another function of spacers may be to prevent the migration of dopants from the source and drain regions (and halo sections) upward into overlying layers (e.g., oxide layers such as the gate oxide layer, etc.). When dopants (e.g., boron) migrate upward it sets up concentration gradients in the underlying source or drain region, which can cause parasitic junctions that increase power consumption by the transistor. Spacers act as a barrier to this type of dopant migration. For example, spacers that include carbon-doped materials are effective in blocking boron from migrating out of underlying source or drain regions.
Conventional spacers are made up of SiO2/Si3N4 layers manufactured by low pressure chemical vapor deposition processes (LPCVD). In such LPCVD methods, a nitrogen-contained gas is reacted with a silicon-containing gas to deposit silicon-nitride on the substrate. Typically, the silicon-containing gas is SiH2Cl2, Si2H6 or SiH4; and the nitrogen-contained gas comes from ammonia (NH3). LPCVD processes typically occur at operating temperature of about 600° C. to about 800° C.
In semiconductor device manufacturing, the conductivity of a semiconductor material may be controlled by doping the semiconductor material with a dopant. The dopant source concentration and distribution affect the performance of the semiconductor devices. At high temperatures, thermal diffusion can cause the dopant region to expand or shift, thereby reducing the concentration of dopant in a dopant region. Extensive thermal diffusion can also cause the dopant region to close or overlap each other causing short channel and punch-through effects. The hydrogen produced when forming a silicon nitride layer by LPCVD may easily be adsorbed by the silicon nitride layer to act like a dopant material. At high temperatures, the hydrogen diffuses into the gate oxide and channel causing a threshold voltage shift of the MOS transistor.
Thus, it is desirable to be able to form spacers that reduce hydrogen atom diffusion into the gate oxide and channel and to reduce adverse electrical effects arising from impurity diffusion, especially for manufacturing highly integrated sub-micron semiconductor processes. However, with conventional methods of forming the SiO2/Si3N4 layers (i.e., LPCVD) it is difficult to control the adverse electrical effects that arise from impurity diffusion. It is also desirable to have a method of manufacturing the SiO2/Si3N4 layers having carbon-doping to slow the diffusion of dopants such as boron. These and other problems are addressed by embodiments of the present invention.